Today many mobile telephones and handheld devices include an RDS (Radio Data System) FM radio together with other main or subsidiary functions. The radio is usually implemented on a chip with only one radio tuner.
In RDS an AF, Alternative Frequencies, list is sent to radio receivers tuned in to a program. The list of alternative frequencies give information on the various transmitters broadcasting the same program in the same or adjacent reception areas, and enable receivers equipped with a memory to store one or more lists to reduce the time for switching to another transmitter.
Furthermore, a PI, Program Identification, code is transmitted together with the program on the channel. This information consists of a code enabling the receiver to distinguish between countries or areas in which the same program is transmitted, and the identification of the program itself. The code is not intended for direct display and is assigned to each individual radio program, to enable it to be distinguished from all other programs. One important application of this information is to enable the receiver to search automatically for an alternative frequency in case of bad reception of the program to which the receiver is tuned; the criteria for the change-over to the new frequency would be the presence of a better signal having the same Program Identification code.
A problem with a handheld device is that the received signal strength may fluctuate rapidly when the device is moving. When the radio senses a low signal this triggers a search for a better channel with the same program. If the device only contains one tuner the radio will be silent during the search. This results in repeated interruptions inconveniencing the listener. Also, the fluctuation may be of such short duration, that a change of channel is not really necessary.